I believe the substance of those suits was on the copyright protection IBM were due on the ROM BIOS image, which was the most substantial piece of IP developed

I believe the substance of those suits was on the copyright protection IBM were due on the ROM BIOS image, which was the most substantial piece of IP developed by IBM for the PC. In the end, clones won because the BIOS call interface could be replicated without directly copying that software artefact -- merely its behaviour -- and so actually seems pretty relevant to API copyright issues generally.